In recent years, a device pattern size has been miniaturized, so that there is an increasing need to control a line width of a pattern formed on a substrate by a lithography process with high accuracy. Specially, in the lithography process for a memory or the like, it is needed to form a high-density line & space pattern under an optical condition close to a resolution limit of an exposure apparatus, so that a focus margin is small.
For example, a step & scan type projection exposure apparatus performs an exposure process on a wafer while performing a focus leveling correction. When the step & scan type projection exposure apparatus exposes, especially, a defective shot (imperfect shot in which a part in a shot is not formed as a product pattern) of a wafer peripheral portion, an effective focus boundary is set at a predetermined distance from the wafer peripheral portion. Then, height information on the wafer is obtained in a region on the inner side of the effective focus boundary, and a focus leveling control is performed by using this height information.
Because the wafer subjected to the actual process has a structure in which a plurality of films is laminated, a film thickness value is different depending on a process factor between a central portion and a peripheral portion of the wafer. Therefore, the effective focus boundary needs to be determined for each process of the exposure process.
However, the defective shot in the peripheral portion of the wafer becomes a shot (hereinafter, dummy shot) in which no device chip can be obtained from the defective shot in some cases. The dummy shot is preferably not exposed for improving throughput of the exposure apparatus; however, if there is no dummy shot, characteristics of an effective chip adjacent to the dummy shot are degraded in the process, such as a dry etching process or a CMP process, other than the lithography.
In a scanning exposure method described in Japanese Patent Application Laid-open No. H08-37150, auto focusing of the wafer is performed based on each focus position measured in a measurement region and a prefetch region on the front side of the measurement region in an exposure field. Then, when an absolute value of the difference between a prefetched focus position and a focus position in the measurement region exceeds an allowable value, prefetched data is ignored and the focus is controlled based on the focus position that is measured in advance at a central portion of the measurement region.
However, in the scanning exposure method described in Japanese Patent Application Laid-open No. H08-37150, when the peripheral portion of the wafer has a step that causes defocusing, a defect failure such as a pattern collapse occurs in some cases by the defocusing. Therefore, a problem arises in that exposure of the dummy shot lowers yield of a peripheral chip.